This invention relates to vacuum pumps known as sputter ion pumps and, more particularly, to a magnet assembly which provides improved sputter ion pump performance.
The basic structure of a sputter ion pump includes an anode, a cathode, and a magnet. The anode includes one or more pump cells, which may be cylindrical. Cathode plates, typically titanium, are positioned on opposite ends of the pump cells. A magnet assembly produces a magnetic field oriented along the axis of the anode. A voltage, typically 3 kV to 9 kV, applied between the cathode plates and the anode produces an electric field which causes electrons to be emitted from the cathode. The magnetic field produces long, more or less helical electron trajectories. The relatively long helical trajectories of the electrons before reaching the anode improves the chances of collision with gas molecules inside the pump cells. When an electron collides with a gas molecule, it tends to liberate another electron from the molecule. The positive ions travel to the cathode due to the action of the electric field. The collision with the solid surface produces a phenomenon called sputtering, i.e., ejection of titanium atoms from the cathode surface. Some of the ionized molecules or atoms impact the cathode surface with sufficient force to penetrate the solid and to remain buried.
Prior art sputter ion pumps have generally satisfactory performance. However, ion pumps typically exhibit decreased pumping speeds at low pressures. Furthermore, ion pumps may extinguish and provide no pumping action at all at very low pressures. The pumping speed of an anode pump cell varies depending on several parameters, including magnetic field strength.
Accordingly, there is a need for improved sputter ion pumps and for magnet assemblies for sputter ion pumps.
According to a first aspect of the invention, a magnet assembly is provided for use in an ion pump. The magnet assembly comprises a magnet yoke having first and second sides and first and second ends which define an interior region, primary magnets of opposite polarities disposed on the first and second ends of the magnet yoke, and secondary magnets disposed on the first and second sides of the magnet yoke.
The secondary magnets may comprise magnets of opposite polarities disposed on the first side of the magnet yoke and magnets of opposite polarities disposed on the second side of the magnet yoke. Each of the secondary magnets is located adjacent to a primary magnet of like polarity.
The magnet assembly may be utilized with any sputter ion pump configuration. For example, the magnet assembly may be utilized with diode ion pumps and triode ion pumps. Furthermore, the magnet assembly may be utilized with ion pumps having any anode cell configuration.
According to another aspect of the invention, an ion pump comprises one or more anode pump cells, a cathode positioned in proximity to the one or more anode pump cells and a magnet assembly for producing a magnetic field in the one or more anode pump cells. An electric field is applied between the cathode and the one or more anode pump cells. The magnet assembly comprises primary magnets of opposite polarities disposed on opposite ends of the anode pump cells, and secondary magnets disposed on opposite sides of the anode pump cells.
According to a further aspect of the invention, a method is provided for operating an ion pump that includes one or more anode pump cells and a cathode. The method comprises applying an electric field between the cathode and the one or more anode pump cells, and producing a magnetic field in the anode pump cells with a magnet assembly including primary magnets on opposite ends of the anode pump cells and secondary magnets on opposite sides of the anode pump cells.